Polyimide films and electronic devices

ABSTRACT

In a first aspect, a polyimide film includes a dianhydride, a fluorinated aromatic diamine and an aliphatic diamine. The polyimide film has a b* of less than one for a film thickness of at least 30 microns and a glass transition temperature of less than 300° C. In a second aspect, an electronic device includes the polyimide film of the first aspect.

FIELD OF DISCLOSURE

The field of this disclosure is polyimide films and electronic devices.

BACKGROUND OF THE DISCLOSURE

Polyimide films can potentially replace rigid glass cover sheets andother substrates which are currently used in display applications, suchas organic light-emitting diode (OLED) displays. For example, aromaticpolyimides are typically very thermally stable, with glass transitiontemperatures (T_(g)) of greater than 320° C., and have excellentfoldability and rollability, a critical property needed fornext-generation flexible displays. For polyimide films used in displayapplications, in addition to having high transmittance and low haze, thepolyimide film also needs to be neutral in color. Typical specificationsrequire that both a* and b* are no greater than 1 color unit fromneutral (0) in CIE L*, a*, b* color space coordinates, i.e., theabsolute values of a* and b* should be less than 1. The threecoordinates of CIE L*, a*, b* represent: (1) the lightness of the color(L*=0 yields black and L*=100 indicates diffuse white), (2) its positionbetween red/magenta and green (negative a* values indicate green, whilepositive values indicate magenta) and (3) its position between yellowand blue (negative b* values indicate blue and positive values indicateyellow).

Typical polyimides with fluorinated monomers, which are nearlycolorless, still absorb light in the blue or violet wavelengths (400-450nm) which gives the films a yellow appearance in transmission. The colorof the polyimide films is mostly generated from charge transferabsorptions arising from HOMO-LUMO transitions which can occur bothwithin the polymer chains and between polymer chains. Various approacheshave been used to alter HOMO-LUMO transition energies or to frustrateinterchain interactions. In one approach, a fluorinated monomer is usedto alter the HOMO-LUMO transition energies of the polyimide polymer, butstill some residual yellow color can be apparent in these polyimidefilms. Depending on the monomer composition in the polyimide, therefore,b* can be higher than 1. Since the CIE L*, a*, b* color measurement of afilm is also dependent on its thickness, achieving a neutral appearanceis even more difficult for thicker films, such as those greater than 25μm.

Polyimides are generally stiff, highly aromatic materials; and thepolymer chains tend to orient in the plane of the film as the film isbeing formed. This leads to differences in the refractive index in theplane of the film and perpendicular to the film. The difference in theparallel and perpendicular refractive indices results in opticalretardation that can negatively impact display performance. Thisdifference is measured as the retardation value (R_(th)) and isrepresented by the equation:R _(th)=[(nx′+ny′)/2−nz′]×d

In the equation above, nx′, ny′ and nz′ respectively indicate refractiveindices in an X-axis direction, a Y-axis direction and a Z-axisdirection in the film. The X axis corresponds to an axial directionexhibiting the refractive index in the surface of the film, the Y axiscorresponds to an axial direction perpendicular to the X axis within thesurface, and the Z axis corresponds to a thickness directionperpendicular to the X axis and the Y axis. Further, d indicates thethickness of the protective layer in nm.

Touch Sensor Panel (TSP) substrates, positioned behind a circularpolarizer in an OLED display, must have ultra-low optical retardation(R_(th)) in addition to visible transparency. Unlike polyimide films forOLED cover sheets, where high T_(g)'s are desirable, the polyimide filmsfor TSP substrates often need to be thermoformable (low T_(g)) whilestill maintaining low color. The thermoformability allows themanufactures to melt process the display components into the desireddevice form which can wrap around the device contours. Unfortunately,synthetic strategies employed to reduce color in polyimide films alsotypically raise their T_(g).

SUMMARY

In a first aspect, a polyimide film includes a dianhydride, afluorinated aromatic diamine and an aliphatic diamine. The polyimidefilm has a b* of less than one for a film thickness of at least 30microns and a glass transition temperature of less than 300° C.

In a second aspect, an electronic device includes the polyimide film ofthe first aspect.

The foregoing general description and the following detailed descriptionare exemplary and explanatory only and are not restrictive of theinvention, as defined in the appended claims.

DETAILED DESCRIPTION

In a first aspect, a polyimide film includes a dianhydride, afluorinated aromatic diamine and an aliphatic diamine. The polyimidefilm has a b* of less than one for a film thickness of at least 30microns and a glass transition temperature of less than 300° C.

In one embodiment of the first aspect, the dianhydride includes4,4′-(hexafluoroisopropylidene)diphthalic anhydride. In a specificembodiment, the dianhydride further includes an alicyclic dianhydride.In a more specific embodiment, the alicyclic dianhydride is selectedfrom the group consisting of cyclobutane dianhydride, cyclohexanedianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride,hexahydro-4,8-ethano-1H,3H-benzo[1,2-c:4,5-c′]difuran-1,3,5,7-tetrone,3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic acid1,4:2,3-dianhydride and meso-butane-1,2,3,4-tetracarboxylic aciddianhydride.

In another embodiment of the first aspect, the fluorinated aromaticdiamine includes 2,2′-bis(trifluoromethyl) benzidine.

In still another embodiment of the first aspect, the aliphatic diamineis selected from the group consisting of 1,2-diaminoethane,1,6-diaminohexane, 1,4-diaminobutane, 1,7-diaminoheptane,1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane (DMD),1,11-diaminoundecane, 1,12-diaminododecane (DDD),1,16-hexadecamethylenediamine,1,3-bis(3-aminopropyl)-tetramethyldisiloxane, isophoronediamine,bicyclo[2.2.2]octane-1,4-diamine and mixtures thereof.

In yet another embodiment of the first aspect, the fluorinated aromaticdiamine is present in a range of from 40 to 95 mole percent, based onthe total diamine content of the polyimide. In a specific embodiment,the fluorinated aromatic diamine is present in a range of from 50 to 75mole percent, based on the total diamine content of the polyimide.

In still yet another embodiment of the first aspect, the polyimide filmhas a thickness in the range of from 10 to 80 μm. In a specificembodiment, the polyimide film has a thickness in the range of from 10to 25 μm.

In a further embodiment of the first aspect, the polyimide film has atransmittance of at least 90 percent, when measured using ASTM D1003over the wavelength range of 400-700 nm.

In still a further embodiment of the first aspect, the polyimide filmhas a haze of less than 1%.

In yet a further embodiment of the first aspect, the polyimide film hasan R_(th) of less than 60 nm.

In a second aspect, an electronic device includes the polyimide film ofthe first aspect.

In one embodiment of the second aspect, the polyimide film is used indevice components selected from the group consisting of substrates forcolor filter sheets, cover sheets, and touch sensor panels.

Many aspects and embodiments have been described above and are merelyexemplary and not limiting. After reading this specification, skilledartisans appreciate that other aspects and embodiments are possiblewithout departing from the scope of the invention. Other features andadvantages of the invention will be apparent from the following detaileddescription, and from the claims.

Depending upon context, “diamine” as used herein is intended to mean:(i) the unreacted form (i.e., a diamine monomer); (ii) a partiallyreacted form (i.e., the portion or portions of an oligomer or otherpolymer precursor derived from or otherwise attributable to diaminemonomer) or (iii) a fully reacted form (the portion or portions of thepolymer derived from or otherwise attributable to diamine monomer). Thediamine can be functionalized with one or more moieties, depending uponthe particular embodiment selected in the practice of the presentinvention.

Indeed, the term “diamine” is not intended to be limiting (orinterpreted literally) as to the number of amine moieties in the diaminecomponent. For example, (ii) and (iii) above include polymeric materialsthat may have two, one, or zero amine moieties. Alternatively, thediamine may be functionalized with additional amine moieties (inaddition to the amine moieties at the ends of the monomer that reactwith dianhydride to propagate a polymeric chain). Such additional aminemoieties could be used to crosslink the polymer or to provide otherfunctionality to the polymer.

Similarly, the term “dianhydride” as used herein is intended to mean thecomponent that reacts with (is complimentary to) the diamine and incombination is capable of reacting to form an intermediate (which canthen be cured into a polymer). Depending upon context, “anhydride” asused herein can mean not only an anhydride moiety per se, but also aprecursor to an anhydride moiety, such as: (i) a pair of carboxylic acidgroups (which can be converted to anhydride by a de-watering orsimilar-type reaction); or (ii) an acid halide (e.g., chloride) esterfunctionality (or any other functionality presently known or developedin the future which is) capable of conversion to anhydridefunctionality.

Depending upon context, “dianhydride” can mean: (i) the unreacted form(i.e. a dianhydride monomer, whether the anhydride functionality is in atrue anhydride form or a precursor anhydride form, as discussed in theprior above paragraph); (ii) a partially reacted form (i.e., the portionor portions of an oligomer or other partially reacted or precursorpolymer composition reacted from or otherwise attributable todianhydride monomer) or (iii) a fully reacted form (the portion orportions of the polymer derived from or otherwise attributable todianhydride monomer).

The dianhydride can be functionalized with one or more moieties,depending upon the particular embodiment selected in the practice of thepresent invention. Indeed, the term “dianhydride” is not intended to belimiting (or interpreted literally) as to the number of anhydridemoieties in the dianhydride component. For example, (i), (ii) and (iii)(in the paragraph above) include organic substances that may have two,one, or zero anhydride moieties, depending upon whether the anhydride isin a precursor state or a reacted state. Alternatively, the dianhydridecomponent may be functionalized with additional anhydride type moieties(in addition to the anhydride moieties that react with diamine toprovide a polymer). Such additional anhydride moieties could be used tocrosslink the polymer or to provide other functionality to the polymer.

Any one of a number of polyimide manufacturing processes may be used toprepare polyimide films. It would be impossible to discuss or describeall possible manufacturing processes useful in the practice of thepresent invention. It should be appreciated that the monomer systems ofthe present invention are capable of providing the above-describedadvantageous properties in a variety of manufacturing processes. Thecompositions of the present invention can be manufactured as describedherein and can be readily manufactured in any one of many (perhapscountless) ways of those of ordinarily skilled in the art, using anyconventional or non-conventional manufacturing technology.

Although methods and materials similar or equivalent to those describedherein can be used in the practice or testing of the present invention,suitable methods and materials are described herein.

When an amount, concentration, or other value or parameter is given aseither a range, preferred range or a list of upper preferable values andlower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

In describing certain polymers, it should be understood that sometimesapplicants are referring to the polymers by the monomers used to makethem or the amounts of the monomers used to make them. While such adescription may not include the specific nomenclature used to describethe final polymer or may not contain product-by-process terminology, anysuch reference to monomers and amounts should be interpreted to meanthat the polymer is made from those monomers or that amount of themonomers, and the corresponding polymers and compositions thereof.

The materials, methods, and examples herein are illustrative only and,except as specifically stated, are not intended to be limiting.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a method,process, article, or apparatus that comprises a list of elements is notnecessarily limited only those elements but may include other elementsnot expressly listed or inherent to such method, process, article, orapparatus. Further, unless expressly stated to the contrary, “or” refersto an inclusive or and not to an exclusive or. For example, a conditionA or B is satisfied by any one of the following: A is true (or present)and B is false (or not present), A is false (or not present) and B istrue (or present), and both A and B are true (or present).

Also, use of the “a” or “an” are employed to describe elements andcomponents of the invention. This is done merely for convenience and togive a general sense of the invention. This description should be readto include one or at least one and the singular also includes the pluralunless it is obvious that it is meant otherwise.

Organic Solvents

Useful organic solvents for the synthesis of the polymers of the presentinvention are preferably capable of dissolving the polymer precursormaterials. Such a solvent should also have a relatively low boilingpoint, such as below 225° C., so the polymer can be dried at moderate(i.e., more convenient and less costly) temperatures. A boiling point ofless than 210, 205, 200, 195, 190, or 180° C. is preferred.

Solvents of the present invention may be used alone or in combinationwith other solvents (i.e., cosolvents). Useful organic solvents include:N-methylpyrrolidone (NMP), dimethylacetamide (DMAc),N,N′-dimethyl-formamide (DMF), dimethyl sulfoxide (DMSO), tetramethylurea (TMU), diethyleneglycol diethyl ether, 1,2-dimethoxyethane(monoglyme), diethylene glycol dimethyl ether (diglyme),1,2-bis-(2-methoxyethoxy) ethane (triglyme), bis [2-(2-methoxyethoxy)ethyl)] ether (tetraglyme), gamma-butyrolactone, andbis-(2-methoxyethyl) ether, tetrahydrofuran. In one embodiment,preferred solvents include N-methylpyrrolidone (NMP) anddimethylacetamide (DMAc).

Co-solvents can generally be used at about 5 to 50 weight percent of thetotal solvent, and useful such co-solvents include xylene, toluene,benzene, “Cellosolve” (glycol ethyl ether), and “Cellosolve acetate”(hydroxyethyl acetate glycol monoacetate).

Diamines

In one embodiment, a suitable diamine for forming the polyimide film caninclude an aliphatic diamine, such as 1,2-diaminoethane,1,6-diaminohexane, 1,4-diaminobutane, 1,7-diaminoheptane,1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane (DMD),1,11-diaminoundecane, 1,12-diaminododecane (DDD),1,16-hexadecamethylenediamine,1,3-bis(3-aminopropyl)-tetramethyldisiloxane, isophoronediamine,bicyclo[2.2.2]octane-1,4-diamine and combinations thereof. Otheraliphatic diamines suitable for practicing the invention include thosehaving six to twelve carbon atoms or a combination of longer chain andshorter chain diamines so long as both developability and flexibilityare maintained. Long chain aliphatic diamines increase flexibility.

In one embodiment, a suitable diamine for forming the polyimide film canfurther include a fluorinated aromatic diamine, such as2,2′-bis(trifluoromethyl) benzidine (TFMB),trifluoromethyl-2,4-diaminobenzene, trifluoromethyl-3,5-diaminobenzene,2,2′-bis-(4-aminophenyl)-hexafluoro propane,4,4′-diamino-2,2′-trifluoromethyl diphenyloxide,3,3′-diamino-5,5′-trifluoromethyl diphenyloxide,9.9′-bis(4-aminophenyl)fluorene,4,4′-trifluoromethyl-2,2′-diaminobiphenyl,4,4′-oxy-bis-[2-trifluoromethyl)benzene amine] (1,2,4-OBABTF),4,4′-oxy-bis-[3-trifluoromethyl)benzene amine],4,4′-thio-bis-[(2-trifluoromethyl)benzene-amine],4,4′-thiobis[(3-trifluoromethyl)benzene amine],4,4′-sulfoxyl-bis-[(2-trifluoromethyl)benzene amine,4,4′-sulfoxyl-bis-[(3-trifluoromethyl)benzene amine],4,4′-keto-bis-[(2-trifluoromethyl)benzene amine],1,1-bis[4′-(4″-amino-2″-trifluoromethylphenoxy)phenyl]cyclopentane,1,1-bis[4′-(4″-amino-2″-trifluoromethylphenoxy)phenyl]cyclohexane,2-trifluoromethyl-4,4′-diaminodiphenyl ether;1,4-(2′-trifluoromethyl-4′,4″-diaminodiphenoxy)-benzene,1,4-bis(4′-aminophenoxy)-2-[(3′,5′-ditrifluoromethyl)phenyl]benzene,1,4-bis[2′-cyano-3′(“4-aminophenoxy)phenoxy]-2-[(3′,5′-ditrifluoro-methyl)phenyl]benzene(6FC-diamine),3,5-diamino-4-methyl-2′,3′,5′,6′-tetrafluoro-4′-tri-fluoromethyldiphenyloxide,2,2-Bis[4′(4″-aminophenoxy)phenyl]phthalein-3′,5′-bis(trifluoromethyl)anilide(6FADAP) and 3,3′,5,5′-tetrafluoro-4,4′-diamino-diphenylmethane (TFDAM).In a specific embodiment, the fluorinated diamine is2,2′-bis(trifluoromethyl) benzidine (TFMB). In one embodiment, afluorinated aromatic diamine can be present in a range of from 40 to 95mole percent, based on the total diamine content of the polyimide. In amore specific embodiment, the fluorinated aromatic diamine can bepresent in a range of from 50 to 75 mole percent, based on the totaldiamine content of the polyimide.

In one embodiment, any number of additional diamines can be used informing the polyimide film, including p-phenylenediamine (PPD),m-phenylenediamine (MPD), 2,5-dimethyl-1,4-diaminobenzene,2,5-dimethyl-1,4-phenylenediamine (DPX), 2,2-bis-(4-aminophenyl)propane, 1,4-naphthalenediamine,1,5-naphthalenediamine,4,4′-diaminobiphenyl, 4,4″-diamino terphenyl, 4,4′-diamino benzanilide,4,4′-diaminophenyl benzoate, 4,4′-diaminobenzophenone,4,4′-diaminodiphenylmethane (MDA), 4,4′-diaminodiphenyl sulfide,4,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl sulfone,bis-(4-(4-aminophenoxy)phenyl sulfone (BAPS),4,4′-bis-(aminophenoxy)biphenyl (BAPB), 4,4′-diaminodiphenyl ether(ODA), 3,4′-diaminodiphenyl ether, 4,4′-diaminobenzophenone,4,4′-isopropylidenedianiline, 2,2′-bis-(3-aminophenyl)propane,N,N-bis-(4-aminophenyl)-n-butylamine, N,N-bis-(4-aminophenyl)methylamine, 1,5-diaminonaphthalene, 3,3′-dimethyl-4,4′-diaminobiphenyl,m-amino benzoyl-p-amino anilide, 4-aminophenyl-3-aminobenzoate,N,N-bis-(4-aminophenyl) aniline, 2,4-diaminotoluene, 2,5-diaminotoluene,2,6-diaminotoluene, 2,4-diamine-5-chlorotoluene,2,4-diamine-6-chlorotoluene, 2,4-bis-(beta-amino-t-butyl) toluene,bis-(p-beta-amino-t-butyl phenyl) ether,p-bis-2-(2-methyl-4-aminopentyl) benzene, m-xylylene diamine, andp-xylylene diamine.

Other useful diamines include 1,2-bis-(4-aminophenoxy)benzene,1,3-bis-(4-aminophenoxy) benzene, 1,2-bis-(3-aminophenoxy)benzene,1,3-bis-(3-aminophenoxy) benzene, 1-(4-aminophenoxy)-3-(3-aminophenoxy)benzene, 1,4-bis-(4-aminophenoxy) benzene, 1,4-bis-(3-aminophenoxy)benzene, 1-(4-aminophenoxy)-4-(3-aminophenoxy) benzene,2,2-bis-(4-[4-aminophenoxy]phenyl) propane (BAPP), 2,2′-bis-(4-phenoxyaniline) isopropylidene, 2,4,6-trimethyl-1,3-diaminobenzene and2,4,6-trimethyl-1,3-diaminobenzene.

Dianhydrides

In one embodiment, any number of suitable dianhydrides can be used informing the polyimide film. The dianhydrides can be used in theirtetra-acid form (or as mono, di, tri, or tetra esters of the tetraacid), or as their diester acid halides (chlorides). However, in someembodiments, the dianhydride form can be preferred, because it isgenerally more reactive than the acid or the ester.

Examples of suitable dianhydrides include 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 1,2,5,6-naphthalene tetracarboxylicdianhydride, 1,4,5,8-naphthalene tetracarboxylic dianhydride,2,3,6,7-naphthalene tetracarboxylic dianhydride,2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzimidazole dianhydride,2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzoxazole dianhydride,2-(3′,4′-dicarboxyphenyl) 5,6-dicarboxybenzothiazole dianhydride,2,2′,3,3′-benzophenone tetracarboxylic dianhydride,2,3,3′,4′-benzophenone tetracarboxylic dianhydride,3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA),2,2′,3,3′-biphenyl tetracarboxylic dianhydride, 2,3,3′,4′-biphenyltetracarboxylic dianhydride,bicyclo-[2,2,2]-octen-(7)-2,3,5,6-tetracarboxylic-2,3,5,6-dianhydride,4,4′-thio-diphthalic anhydride, bis (3,4-dicarboxyphenyl) sulfonedianhydride, bis (3,4-dicarboxyphenyl) sulfoxide dianhydride (DSDA), bis(3,4-dicarboxyphenyl oxadiazole-1,3,4) p-phenylene dianhydride, bis(3,4-dicarboxyphenyl) 2,5-oxadiazole 1,3,4-dianhydride, bis2,5-(3′,4′-dicarboxydiphenylether) 1,3,4-oxadiazole dianhydride,4,4′-oxydiphthalic anhydride (ODPA), bis (3,4-dicarboxyphenyl) thioether dianhydride, bisphenol A dianhydride (BPADA), bisphenol Sdianhydride, bis-1,3-isobenzofurandione, 1,4-bis(4,4′-oxyphthalicanhydride) benzene, bis (3,4-dicarboxyphenyl) methane dianhydride,cyclopentadienyl tetracarboxylic acid dianhydride, cyclopentanetetracarboxylic dianhydride, ethylene tetracarboxylic acid dianhydride,perylene 3,4,9,10-tetracarboxylic dianhydride, pyromellitic dianhydride(PMDA), tetrahydrofuran tetracarboxylic dianhydride,1,3-bis-(4,4′-oxydiphthalic anhydride) benzene,2,2-bis(3,4-dicarboxyphenyl) propane dianhydride,2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,phenanthrene-1,8,9,10-tetracarboxylic dianhydride,pyrazine-2,3,5,6-tetracarboxylic dianhydride,benzene-1,2,3,4-tetracarboxylic dianhydride andthiophene-2,3,4,5-tetracarboxylic dianhydride.

In one embodiment, a suitable dianhydride can include an alicyclicdianhydride, such as cyclobutane diandydride (CBDA), cyclohexanedianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA),hexahydro-4,8-ethano-1H,3H-benzo[1,2-c:4,5-c′]difuran-1,3,5,7-tetrone(SODA), 3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic acid1,4:2,3-dianhydride (TCA) and meso-butane-1,2,3,4-tetracarboxylic aciddianhydride.

In one embodiment, a suitable dianhydride for forming the polyimide filmcan include a fluorinated dianhydride, such as4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 9,9-bis(trifluoromethyl)-2,3,6,7-xanthene tetracarboxylic dianhydride. In aspecific embodiment, the fluorinated dianhydride is4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA).

Polyimide Films

In one embodiment, a polyimide film can be produced by combining adiamine and a dianhydride (monomer or other polyimide precursor form)together with a solvent to form a polyamic acid (also called a polyamideacid) solution. The dianhydride and diamine can be combined in a molarratio of about 0.90 to 1.10. The molecular weight of the polyamic acidformed therefrom can be adjusted by adjusting the molar ratio of thedianhydride and diamine.

In one embodiment, a polyamic acid casting solution is derived from thepolyamic acid solution. The polyamic acid casting solution preferablycomprises the polyamic acid solution can optionally be combined withconversion chemicals like: (i) one or more dehydrating agents, such as,aliphatic acid anhydrides (acetic anhydride, etc.) and/or aromatic acidanhydrides; and (ii) one or more catalysts, such as, aliphatic tertiaryamines (triethyl amine, etc.), aromatic tertiary amines (dimethylaniline, etc.) and heterocyclic tertiary amines (pyridine, picoline,isoquinoline, etc.). The anhydride dehydrating material it is often usedin molar excess compared to the amount of amide acid groups in thepolyamic acid. The amount of acetic anhydride used is typically about2.0-4.0 moles per equivalent (repeat unit) of polyamic acid. Generally,a comparable amount of tertiary amine catalyst is used. Nanoparticles,dispersed or suspended in solvent as described above, are then added tothe polyamic acid solution.

In one embodiment, the polyamic acid solution, and/or the polyamic acidcasting solution, is dissolved in an organic solvent at a concentrationfrom about 5.0 or 10% to about 15, 20, 25, 30, 35 and 40% by weight.

The polyamic acid (and casting solution) can further comprise any one ofa number of additives, such as processing aids (e.g., oligomers),antioxidants, light stabilizers, flame retardant additives, anti-staticagents, heat stabilizers, ultraviolet absorbing agents, inorganicfillers or various reinforcing agents. Inorganic fillers can includethermally conductive fillers, metal oxides, inorganic nitrides and metalcarbides, and electrically conductive fillers like metals, graphiticcarbon and carbon fibers. Common inorganic fillers are alumina, silica,silicon carbide, diamond, clay, boron nitride, aluminum nitride,titanium dioxide, dicalcium phosphate, and fumed metal oxides. Commonorganic fillers include polyaniline, polythiophene, polypyrrole,polyphenylenevinylene, polydialkylfluorenes, carbon black, graphite,multiwalled and single walled carbon nanotubes and carbon nanofibers.

The solvated mixture (the polyamic acid casting solution) can then becast or applied onto a support, such as an endless belt or rotatingdrum, to give a film. In one embodiment, the polyamic acid can besolution cast in the presence of an imidization catalyst. Use of animidization catalyst can help to lower the imidization temperature andshorten the imidization time, and can also help in the formation ofrefractive index-matching nanoparticle aggregates that essentiallymaintain the volume ratio of low and high index nanoparticles in theaggregate. Typical imidization catalysts can range from bases such asimidazole, 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole,2-phenylimidazole, benzimidazole, isoquinoline, or substituted pyridinessuch as methyl pyridines, lutidine, and trialkylamines. Combinations ofthe tertiary amines with acid anhydrides can be used. These dehydrationagents, which can act as co-catalysts, include acetic anhydride,propionic anhydride, n-butyric anhydride, benzoic anhydride and others.The ratio of these catalysts and their concentration in the polyamicacid layer will influence imidization kinetics and the film properties.Next, the solvent containing-film can be converted into aself-supporting film by heating at an appropriate temperature (thermalcuring) together with conversion chemical reactants (chemical curing).The film can then be separated from the support, oriented such as bytentering, with continued thermal and chemical curing to provide apolyimide film.

Useful methods for producing polyimide films containing a polyimide inaccordance with the present invention can be found in U.S. Pat. Nos.5,166,308 and 5,298,331, which are incorporate by reference into thisspecification for all teachings therein. Numerous variations are alsopossible, such as,

-   -   (a) A method wherein the diamine components and dianhydride        components are preliminarily mixed together and then the mixture        is added in portions to a solvent while stirring.    -   (b) A method wherein a solvent is added to a stirring mixture of        diamine and dianhydride components. (contrary to (a) above)    -   (c) A method wherein diamines are exclusively dissolved in a        solvent and then dianhydrides are added thereto at such a ratio        as allowing to control the reaction rate.    -   (d) A method wherein the dianhydride components are exclusively        dissolved in a solvent and then amine components are added        thereto at such a ratio to allow control of the reaction rate.    -   (e) A method wherein the diamine components and the dianhydride        components are separately dissolved in solvents and then these        solutions are mixed in a reactor.    -   (f) A method wherein the polyamic acid with excessive amine        component and another polyamic acid with excessive dianhydride        component are preliminarily formed and then reacted with each        other in a reactor, particularly in such a way as to create a        non-random or block copolymer.    -   (g) A method wherein a specific portion of the amine components        and the dianhydride components are first reacted and then the        residual diamine components are reacted, or vice versa.    -   (h) A method wherein the conversion chemicals (catalysts) are        mixed with the polyamic acid to form a polyamic acid casting        solution and then cast to form a gel film.    -   (i) A method wherein the components are added in part or in        whole in any order to either part or whole of the solvent, also        where part or all of any component can be added as a solution in        part or all of the solvent.    -   (j) A method of first reacting one of the dianhydride components        with one of the diamine components giving a first polyamic acid.        Then reacting another dianhydride component with another amine        component to give a second polyamic acid. Then combining the        amic acids in any one of a number of ways prior to film        formation.

In one embodiment, if the polyimide is soluble in a non-protic solvent,such as DMAc or NMP, the polyimide can be formed in solution, optionallywith the addition of catalysts at higher temperatures (>25° C.). Afterfiltration, the polyimide powder can be re-dissolved in a solvent. Thepolyimide solution can then be cast onto a support (e.g. a moving beltor rigid support) and coalesced to create a polyimide film.

The thickness of the polymer film may be adjusted, depending on theintended purpose of the film or final application specifications. In oneembodiment, the polyimide film has a total thickness in a range of fromabout 10 to about 80 μm, or from about 10 to about 25 μm, or from about15 to about 25 μm.

Applications

In one embodiment, a polyimide film can be used for a number of layersin electronic device applications, such as in an organic electronicdevice. Nonlimiting examples of such layers include device substrates,touch panels, substrates for color filter sheets, cover films, andothers. The particular materials' properties requirements for eachapplication are unique and may be addressed by appropriatecomposition(s) and processing condition(s) for the polyimide filmsdisclosed herein. Organic electronic devices that may benefit fromhaving a coated film include, but are not limited to, (1) devices thatconvert electrical energy into radiation (e.g., a light-emitting diode,light emitting diode display, lighting device, luminaire, or diodelaser), (2) devices that detect signals through electronics processes(e.g., photodetectors, photoconductive cells, photoresistors,photoswitches, phototransistors, phototubes, IR detectors, biosensors),(3) devices that convert radiation into electrical energy, (e.g., aphotovoltaic device or solar cell), (4) devices that convert light ofone wavelength to light of a longer wavelength, (e.g., a down-convertingphosphor device); and (5) devices that include one or more electroniccomponents that include one or more organic semi-conductor layers (e.g.,a transistor or diode).

In one embodiment, a polyimide film can be used as a touch sensor panel(TSP) substrate. TSP substrates, positioned behind a circular polarizerin an OLED display, must have ultra-low optical retardation (R_(th)) inaddition to visible transparency. In one embodiment, a polyimide filmcan have a b* of less than one for a film thickness of at least 30microns and a glass transition temperature of less than 300° C. In aspecific embodiment, the polyimide film has a T_(g) of less than 275°C., or less than 250° C. In one embodiment, a polyimide film can have anR_(th) of less than 60 nm, or less than 50 nm, or less than 40 nm. Inone embodiment, a polyimide film can have a transmittance of at least90%, when measured using ASTM D1003 over the wavelength range of 400-700nm. In one embodiment, a polyimide film can have a haze of less than 1%.In some instances, TSP substrates need to be thermoformable (low T_(g))while maintaining low color. The thermoformability allows themanufactures to melt process the display components into the desireddevice form which can wrap around the device contours.

The advantageous properties of this invention can be observed byreference to the following examples that illustrate, but do not limit,the invention. All parts and percentages are by weight unless otherwiseindicated.

EXAMPLES Test Methods

Measurement of CIE L*, a*, b* Color

Color measurements were performed using a ColorQuest® XE dual-beamspectrophotometer (Hunter Associates Laboratory, Inc., Reston, Va.),using D65 illumination and 10 degree observer, in total transmissionmode over a wavelength range of 380 to 780 nm.

Transmittance and Haze

Transmittance and haze were measured using a Haze-Guard Plus(BYK-Gardner GmbH, Germany), with the haze measured in transmission bycollecting forward scattered light using the method described byASTM1003. Percent haze was determined by measuring the amount of lightwhich deviates from the incident beam by more than 2.5 degrees onaverage.

Glass Transition Temperature

Glass transition temperature (T_(g)) was measured using a DMA Q800 (TAInstruments, New Castle, Del.) operating at a frequency of 1 Hz over atemperature range of from room temperature to 350° C.

Through-Plane Retardation

Through plane retardation (R_(th)) was measured at 550 nm using anAxoScan™ Mapping SpectroPolarimeter (Axometrics Inc., Huntsville, Ala.).

Thickness

Coating thickness was determined by measuring coated and uncoatedsamples in 5 positions across the profile of the film using acontact-type FISCHERSCOPE MMS PC2 modular measurement system thicknessgauge (Fisher Technology Inc., Windsor, Conn.).

Example 1

For the polyamic acid solution (PAA) of Example 1 (E1) with a monomercomposition of 6FDA 1.0//TFMB 0.5/HMD 0.5 (molar equivalents), into a500-ml reaction vessel, equipped with mechanical stirring and nitrogenpurged atmosphere, 298.2 g anhydrous DMAc and 50.0 g of4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA, Synasia Inc.,Metuchen, N.J.) was added. 18.02 g of trifluoromethylbenzidine (TFMB,Seika Corp., Wakayam Seika Kogyo Co., LTD., Japan) and 6.54 g of1,6-diaminohexane (HMD, TCI America, Portland, Oreg.) were added slowlyover a period of 20 minutes. The reaction mixture was stirred and heatedat 40° C. for 16 hours. The solution became slightly viscous. Films wereprepared by doctor blade coating the polyamic-acid solution onto glasstreated with a release agent. The polyamic-acid films were dried at 80°C. to form a film of approximately 70 to 80 wt % solids. The film wasthen cured in an oven from 150° C. to 300° C. over the course of 20minutes. The dry thickness of the film was 34.0 μm.

Example 2

For Example 2 (E2), the same procedure as described in E1 was used, butthe monomer composition was 6FDA 1.0//TFMB 0.75/HMD 0.25. The drythickness of the film was 36.8 μm.

Both E1 and E2 had an excellent combination of low color and T_(g), goodtransmittance and haze, and low optical retardation.

Comparative Example 1

For Comparative Example 1 (CE1), the same procedure as describe in E1was used, but the monomer composition was 6FDA 1.0//TFMB 1.0. The drythickness of the film was 32.1 μm. With a fluorinated aromatic diamine,but no aliphatic diamine, CE1 has good optical properties, but with ahigher T_(g).

Comparative Example 2

For Comparative Example 2 (CE2), the same procedure as describe in E1was used, but the monomer composition was 6FDA1.0//3,5-diaminobenzotrifluoride, a meta-diamine with a trifluoromethylgroup (Oakwood Chemical, Estill, S.C.). The dry thickness of the filmwas 34.4 μm. Despite the presence of the meta-linkage and the electronwithdrawing trifluoromethyl group, the b* of CE2 was significantlyhigher at 2.47.

Comparative Example 3

For Comparative Example 3 (CE3), the same procedure as describe in E1was used, but the monomer composition was 6FDA 1.0//TFMB0.5/1,4-butanediamine 0.5 (Sigma-Aldrich, St. Louis, Mo.). The drythickness of the film was 38.6 μm. The film was very brittle and had ahigh b* of 2.02.

Comparative Example 4

For Comparative Example 4 (CE4), the same procedure as describe in E1was used, but the monomer composition was 6FDA 1.0//TFMB0.5/2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane 0.5 (BDAF,Fluorotech USA, Rancho Cucamonga, Calif.). The dry thickness of the filmwas 38.1 μm. Despite the inclusion of a flexible fluorinated aromaticdiamine, the b* of CE4 was quite high at 3.67.

As shown in Table 1, E1 and E2 provide good optical properties, with lowcolor (b*) and low optical retardation (R_(th)), while also havingsignificantly lower glass transition temperatures. Their low T_(g)'senable them to be thermoformable, and thus they can be used aselectronic device layers where thermoformability is needed, such as in atouch sensor panel.

TABLE 1 Thickness T_(g) Transmittance Haze R_(th) Example (μm) b* (° C.)(%) (%) (nm) E1 34.0 0.70 181 90 0.18 34.0 E2 36.8 0.89 240 90 0.19 27.6CE1 32.1 1.16 325 90 0.32 28.8 CE2 34.4 2.47 300 89 — 16.3 CE3 38.6 2.02— — — — CE4 38.1 3.67 294 87 — —

Note that not all of the activities described above in the generaldescription are required, that a portion of a specific activity may notbe required, and that further activities may be performed in addition tothose described. Still further, the order in which each of theactivities are listed are not necessarily the order in which they areperformed. After reading this specification, skilled artisans will becapable of determining what activities can be used for their specificneeds or desires.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the invention as set forth in theclaims below. All features disclosed in this specification may bereplaced by alternative features serving the same, equivalent or similarpurpose.

Accordingly, the specification and figures are to be regarded in anillustrative rather than a restrictive sense and all such modificationsare intended to be included within the scope of the invention.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims.

What is claimed is:
 1. A polyimide film comprising a polyimide derivedfrom: a dianhydride; a fluorinated aromatic diamine; and an aliphaticdiamine, wherein the polyimide film has: a b* of less than one for afilm thickness of at least 30 microns; and a glass transitiontemperature of less than 300° C.
 2. The polyimide film of claim 1,wherein the dianhydride comprises4,4′-(hexafluoroisopropylidene)diphthalic anhydride.
 3. The polyimidefilm of claim 2, wherein the dianhydride further comprises an alicyclicdianhydride.
 4. The polyimide film of claim 3, wherein the alicyclicdianhydride is selected from the group consisting of cyclobutanedianhydride, cyclohexane dianhydride,1,2,3,4-cyclopentanetetracarboxylic dianhydride,hexahydro-4,8-ethano-1H,3H-benzo[1,2-c:4,5-c]difuran-1,3,5,7-tetrone,3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic acid1,4:2,3-dianhydride and meso-butane-1,2,3,4-tetracarboxylic aciddianhydride.
 5. The polyimide film of claim 1, wherein the fluorinatedaromatic diamine comprises 2,2′-bis(trifluoromethyl) benzidine.
 6. Thepolyimide film of claim 1, wherein the aliphatic diamine is selectedfrom the group consisting of 1,2-diaminoethane, 1,6-diaminohexane,1,4-diaminobutane, 1,7-diaminoheptane, 1,8-diaminooctane,1,9-diaminononane, 1,10-diaminodecane (DMD), 1,11-diaminoundecane,1,12-diaminododecane (DDD), 1,16-hexadecamethylenediamine,1,3-bis(3-aminopropyl)-tetramethyldisiloxane, isophoronediamine,bicyclo[2.2.2]octane-1,4-diamine and mixtures thereof.
 7. The polyimidefilm of claim 1, wherein the fluorinated aromatic diamine is present ina range of from 40 to 95 mole percent, based on the total diaminecontent of the polyimide.
 8. The polyimide film of claim 7, wherein thefluorinated aromatic diamine is present in a range of from 50 to 75 molepercent, based on the total diamine content of the polyimide.
 9. Thepolyimide film of claim 1, wherein the polyimide film has a thickness inthe range of from 10 to 80 μm.
 10. The polyimide film of claim 9,wherein the polyimide film has a thickness in the range of from 10 to 25μm.
 11. The polyimide film of claim 1, wherein the polyimide film has atransmittance of at least 90 percent, when measured using ASTM D1003over the wavelength range of 400-700 nm.
 12. The polyimide film of claim1, wherein the polyimide film has a haze of less than 1%.
 13. Thepolyimide film of claim 1, wherein the polyimide film has an R_(th) ofless than 60 nm.
 14. An electronic device comprising the polyimide filmof claim
 1. 15. The electronic device of claim 14, wherein the polyimidefilm is used in device components selected from the group consisting ofsubstrates for color filter sheets, cover sheets, and touch sensorpanels.
 16. The polyimide film of claim 1, wherein the polyimide filmhas a glass transition temperature of less than 275° C.
 17. Thepolyimide film of claim 16, wherein the polyimide film has a glasstransition temperature of less than 250° C.
 18. The polyimide film ofclaim 13, wherein the polyimide film has an R_(th) of less than 50 nm.19. The polyimide film of claim 18, wherein the polyimide film has anR_(th) of less than 40 nm.